1. Field of the Invention
The present invention relates generally to sound capture, and particularly to capturing a user's voice sounds.
2. State of the Art
Some users require devices such as voice pickups that provide a high quality capture of sound, for example, of the user's speech, and that are compact, discreet and convenient. For example, U.S. Secret Service agents assigned to covertly protect individuals require two way communications systems that are unobtrusive and allow them to clearly communicate with remote locations. Such an application requires a sound pickup that an agent can wear which is both discreet and accurate, particularly in situations such as a crowded room where ambient sound levels are high. Conventional two way communications systems used in such applications typically have boom-mounted or lapel-mounted microphones. Boom-mounted microphones are difficult to conceal, and lapel-mounted microphones and other conventional microphones mounted remotely from the user's mouth often suffer from a relatively low signal-to-noise ratio, especially in noisy environments.
One way that has been tried for improving the signal-to-ambient noise ratio has been to employ directional microphones. For example, miniature directional microphones have been used that are pressure gradient microphones which have been modified by the incorporation of delay networks that include an acoustic delay line into the rear port, as shown for example in FIG. 2 of the Knowles Electronics, Inc. Technical Bulletin TB21, which is hereby incorporated by reference. As described in Bulletin TB21, use of a one-port delay produces useful directional polar patterns, which can be tuned by selecting different ratios of front-to-back port spacing and selecting the rear port acoustic delay.
In these microphones, the necessary acoustic delay is formed by a combination of the compliance, created by the volume of air trapped in the microphone element, the acoustic inertance due to the mass of air in the port opening, and an acoustic resistance provided by an acoustic damper added to the port as described for example in “Subminiature Directional Microphones” in a paper by Elmer V. Carlson and Mead C. Killion, presented on Sep. 10, 1973 at the Convention of the Audio Engineering Society in New York, and which is hereby incorporated by reference. Such an acoustic network can be made to produce an acoustic delay which is very constant over frequency and which does not attenuate the delayed signal over the design frequency bandwidth. In addition, the front port is intentionally designed to minimize both its acoustic delay and attenuation characteristics. These properties are critically necessary for the microphone to have the desired directional characteristics, which do not vary with frequency. However, they still suffer from sound quality and signal-to-ambient noise ratio problems, and tend to be relatively large and noticeable.
Accordingly, there is a need for a voice pickup device and method that are both highly discreet, and capable of capturing a user's speech with a high sound quality and a relatively large signal-to-ambient noise ratio.